The present invention generally relates to wireless communication systems, and particularly relates to determination of the initial transmit power required to reliably establish a forward link traffic channel between a network and a mobile station.
Transmit power control is an essential element of many wireless communication systems, and is particularly needed in Code Division Multiple Access (CDMA) systems, where potentially large numbers of users share a common frequency or frequencies and rely on unique spreading codes to differentiate between users' signals. Such systems employ relatively sophisticated power control mechanisms for both forward link and reverse link transmissions.
In the typical CDMA system, forward and reverse link power control rely on closed-loop transmit power control during active communication between the network and a given mobile station. For example, during an active call, the network controls forward link traffic channel transmit power based on power control commands returned from the associated mobile station. If received signal quality at the mobile station falls below a desired threshold, it commands the network to increase transmit power; otherwise it commands the network to decrease transmit power. Thus, forward link transmit power is generally maintained at the minimum level required to maintain acceptable received signal quality at the mobile station. Reverse link power control employs similar closed-loop techniques, but with the network providing power control commands to the mobile stations, such that supporting base stations receive transmissions from the mobile stations at minimum required levels.
While the above power control schemes offer advantageous operation during active calls, they generally require the existence of a traffic channel or other data channel on the forward and reverse links. That is, these closed-loop power control schemes represent an approach to maintaining transmit powers in support of a communication over one or more traffic channels, but they do not, for example, provide a mechanism for establishing or determining an initial transmit power level.
Initialization of forward link traffic channel power arises in several scenarios including, but not limited to, call origination, soft/softer handoff, and hard handoff. Reliable establishment of forward link traffic channels aids network efficiency by reducing the signaling overhead arising from repeated connection attempts, and measurably influences users' perceptions of service quality. Thus, a conventional approach to setting initial traffic channel transmit power embraces a “more is better” philosophy, and simply sets the initial transmit power at a level high enough to ensure reliable mobile station reception under many circumstances.
Of course, once the forward link traffic channel is established, power control feedback from the mobile station generally reduces transmit power to the minimum level required. Still, using a high default initial transmit power means that statistically the initial transmit power is set higher than actually needed by the typical mobile station. As such, opportunities exist for improving overall network efficiency and capacity based on a more intelligent approach to power initialization.